Flow molding means and method

ABSTRACT

Thermosetting molds are formed with electrically conductive surfaces for use in high frequency energy flow molding of sheet materials. Preferably the mold of this invention has a highly electrically conductive layer preferably in an epoxy gel coat carrying electrically conductive particles of gold, silver or platinum and positioned over a compatible epoxy mold body. A vacuum molding technique is used to obtain good surface definition in surface molding.

RELATED APPLICATION

This application is a continuation-in-part of applicants' copendingpatent application Ser. No. 308,830 filed Nov. 22, 1973, and nowabandoned.

BACKGROUND OF THE INVENTION

Flow molding with the use of high frequency energy such as radiofrequency energy has come into widespread use in recent timesparticularly in the surface molding of shoe uppers. As well-known, insuch procedures, a negative RTV silicone rubber mole is produced,positioned on a ground electrode and a sheet stock such as vinyl sheetstock to be surfaced is laid over the mold with a top electrode pressedthereon and radio frequency energy flowed through the mold to thermallysoften the vinyl and allow it to take on the surface characteristics ofthe negative mold.

The molds used are frequently made from RTV silicone rubber in knownflow mold making procedures. However, such RTV silicone rubber moldmasters have limited life spans in high frequency flow molding. The RTVsilicone rubber molds act to absorb heat causing increased dwell timesin the mold and in some cases, absorb plasticizers and secondaryplasticizers of the vinyl thereby causing weakening, swelling anddimensional change of the silicone rubber molds. The definition of thesurface configuration to be transferred is sometimes lost after fewmolding operations. The silicone rubber in some cases tears orpermanently distorts. These defects in the silicone rubber molds used inflow molding are well-known.

It has been suggested that more durable molds be formed. For example,epoxy molds have been suggested. However, it is found that when suchepoxy molds are used in radio frequency flow molding, the epoxy tends toabsorb heat at a rate 3 to 8 times greater than absorbed by siliconerubber thereby lengthening dwell times in the mold to an unacceptabledegree. Moreover, detail of the surface configuration to be transferredis sometimes lost. The removal of heat by the epoxy is such that in somecases the vinyl sheet being molded never reaches a molten state to allowdetail to be transferred.

Probably because of the foregoing problems, the art has in most casescontinued to use RTV silicone mold masters in high frequency flowmolding procedures.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a long life thermosettingmold for use in high frequency flow molding which mold does not absorbheat of molding in an amount to destroy or seriously affect the moldingoperation.

Another object of this invention is to provide a thermosetting plasticmold in accordance with the preceding object which has an electricallyconductive molding surface.

It is still another object of this invention to provide a thermosettingplastic mold in accordance with the preceding objects which is resistantto adverse interaction with sheets being molded, retains transfer ofdefinition for long time periods, is non-self-destructive under normaloperation pressures and temperatures and has many of the advantageouscharacteristics of a metal mold including long life and high definition.

Still another object of this invention is to provide a method of flowmolding incorporating the molds of this invention.

Still another object of this invention is to provide an improved vacuumassist step for use in obtaining good surface definition in flowmolding.

According to the invention, a thermosetting plastic production mold isformed with an electrically conductive molding surface layer. Preferablythe mold is formed with a surface layer of conductive particle filledresin, although in some cases the entire body of the mold can beconductive.

In a preferred method a vacuum is created between a sheet to be surfaceflow molded and the forming mold to enhance surface definition of the somolded sheet.

The electrically conductive surface layer is suitable for grounding inan RF flow molding machine. When used in this manner, no deleteriousheat buildup occurs in the thermosetting plastic mold. High surfacedefinition can be obtained in sheet materials molded over long timeperiods without cracking or shattering of the mold. Because of the highthermal conductivity as well as electrical conductivity of the surfacelayer, short molding cooling cycles and shorter dwell times thancustomarily used are obtained. Because the mold is rigid rather thanflexible as with previous silicone rubber master molds, better detailsuch as detail of thread twist, needle perforation and the like iseasily obtained. The conductive surface is preferably, continuous,liquid impervious and non-porous thus allowing for good surfacedefinition in molding. The molds are rigid at temperatures and pressuresused in conventional flow molding.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features, advantages and objects of the presentinvention will be better understood from the following specificationwhen read in conjunction with the accompanying drawings in which:

FIG. 1 is a top plan view of a shoe upper having surface detail and usedto produce a production mold in accordance with this invention;

FIGS. 2-5 are semidiagrammatic illustrations of steps in the method ofproducing the thermosetting plastic mold of this invention;

FIG. 6 is a side cross sectional view through a thermosetting plasticmold in accordance with this invention;

FIG. 7 is a top plan view thereof; and

FIG. 8 is a semidiagrammatic showing of an improvement in conventionalflow molding methods.

DESCRIPTION OF PREFERRED EMBODIMENTS

The high frequency flow molding conditions and apparatus used are aswell-known in the art. For example, high frequency molding can becarried out at the following conditions:pressure (lbs/sq.in.on 0.5 to1000 lbs/sq.in.sheet molded)dwell time 1-20 secondstemperature70°F-400°Ffrequency 20 - 54 megacyclesWattage 7.0 to 70 kilowattscoolingtime under pressureafter dwell time underelectrical energy 1 to 60seconds______________________________________

In such molding as known, a sheet material such as a vinyl sheet forexample having a thickness of from 0.001 inch to 0.200 inch is placedover a production mold which is in turn placed on a ground electrode anda top electrode brought into contact with the vinyl sheet to press itagainst the production mold while the high frequency energy is passedtherethrough. Slight flow molding occurs causing the sheet material totake on the surface contour and definition of the production mold. Forexample, as known in the shoe art, surface textures including stitching,perforation and the like can be imparted to a vinyl sheet during suchflow molding. The surface contour and texture of a variety of shoe uppermaterials including stitched and embossed leather can be simulated. Insome cases, the sheet to be molded need not be vinyl but other heatsoftening plastics and even surface treated leather can be surface flowmolded by standard procedures.

The particular production mold used in flow molding is an importantfeature of the present invention. It has now been found that by using athermosetting rigid plastic production mold having an electricallyconductive surface, long life, high definition molds can be rapidly andinexpensively made for use in flow molding.

In an example of making a negative thermosetting plastic negativeproduction mold of this invention, a conventional three-step process isused with the third step incorporating the conductive layer of thisinvention. In a first stage, a conventional flow molding mold box asdiagrammatically shown at 10 in FIG. 2 has a rectangular frame 11thereover with a pattern or master 12 positioned therein. The pattern 12as best shown in FIG. 1, can be a leather-shoe upper having an embossedtoe portion 13 with a line of stitching 14 therein and a series ofperforations 15 with a leather grain 16 thereover. The object is toproduce a mold which will reproduce the embossed, perforated and surfacedefinition of the shoe master 12. The first stage is carried out bybonding the master 12 to a flat planar lower surface of the mold box asby the use of a double surface sticky tape which is compatible with theresins later used and which may be Minnesota Mining and ManufacturingCompany two-way tape No. 419. A mold release agent containing anisopropyl-alcohol-water solution of polyvinyl alcohol is then brushed orsprayed onto all surfaces and allowed to dry for 30 minutes. Uncoatedvinyl masters do not require the mold release. Four parts of Arcon epoxyT4051A are then mixed with 1 part Arcon T4051B by weight and degassed ina vacuum chamber having about 4 times the volume of the epoxy inaccordance with known procedures. Arcon epoxy T4051A is a product ofAllied Resin Corporation of East Weymouth, Massachusetts formed of abisphenol A/epichlorohydrin epoxy resin filled with 50% by weight CaCO₃of a particle size ranging from 0.1 to 25 microns with an apparent epoxyequivalent weight of 360 ± 10. Arcon epoxy T4051B is a product of AlliedResin Corporation of East Weymouth, Mass. formed ofpolyoxypropyleneamine having an average molecular weight of 340 ± 50.

The epoxy mixture is then poured slowly over the master 12 and cured for24 to 36 hours at room temperature to produce a negative first stageepoxy mold 20. Curing can be carried out at 110°F for 12 hours ifdesired.

As illustrated in FIG. 3, using the first stage mold 20, the generalprocedure illustrated in FIG. 2 is repeated in a second stage. In thissecond stage, the first stage epoxy mold is bonded to the mold box 10with a two-surface stick tape. Ten parts by weight of a silicone rubberRTV 664A, a trademarked product of General Electric Co. of Waterford,N.Y. containing a high strength methyl silane, addition curing, roomtemperature vulcanizing silicone rubber is mixed with 1 part by weightRTV 664B, a trademarked product of General Electric Co. of Waterford,N.Y., and containing a low molecular weight silicone rubber curingagent. The mixture is degassed in an area approximately 5 times thevolume of the mixture after which the mixture is poured into the moldbox and cured for 24 hours at room temperature or until tack free. Theresulting silicone rubber positive second stage mold 21 is then thusformed and may be postcured for 2 to 3 hours at 300°F or until allsurfaces are tack free. This results in a positive RTV silicone rubbermold 21, which is useful for forming the electrically conductiveproduction mold of this invention.

In a third stage, a production mold of this invention is formed. Threeparts by weight of T4083A, a product of Allied Resin Corporation of EastWeymouth, Mass., formed of 24% by weight of bisphenol A/epichlorohydrinepoxy resin (molecular weight 360 ± 10) filled with 76% by weight ofsilver flake of a particle size ranging from 2 to 25 microns with anapparent epoxy equivalent weight of 700 ± 50 are mixed with 1 part byweight T4083B curing agent, a product of Allied Resin Corporation ofEast Weymouth, Mass., formed of methylethyl ketone solvent solution oftwo isomers 2,2,4 and 2,4,4 trimethylhexamethylenediamine and N-2hydroxypropylimidizol as follows:

    2.79% by weight                                                                            2,2,4 trimethylhexamethylenediamine                              2.79% by weight                                                                            2,4,4 trimethylhexamethylenediamine                              1.72% by weight                                                                            N-2 hydroxypropylimidizol                                        92.7% by weight                                                                            methylethyl ketone                                           

The mixture is then sprayed onto the silicone rubber mold 21 in a moldbox 10 using a Paache No. 3 air brush to develop a film thickness of0.010 inch. The mold box 10 is then heated at 140°F for 2 hours in anair circulating oven. This forms a gel coat of an epoxy over thesilicone rubber positive mold 21. 100 parts of Arcon epoxy T4046A, aproduct of Allied Resin Corporation of East Weymouth, Mass., formed ofan admixture of:

    26.7% by weight                                                                          bisphenol A/epichlorohydrin epoxy resin                                       molecular weight of 360 ± 10                                     5.4% by weight                                                                          milled glass fibers (1/32 wide screen                                         size)                                                              14.6% by weight                                                                          Al.sub.2 O.sub.3 particle size 1 to 8 microns                      53.3% by weight                                                                          Iron powder particle size 5-125 microns.                       

The above admixture, having an apparent epoxy equivalent weight of 675 ±10, is mixed together with 3 parts by weight Arcon T4046B formed of:

    38.3% by weight                                                                            2,2,4 trimethylhexamethylenediamine                              38.3% by weight                                                                            2,4,4 trimethylhexamethylenediamine                              23.4% by weight                                                                            N-2 hydroxypropylimidizol                                    

The mixture is degassed in a mixing container having a volumeapproximately 5 times the volume of the mixture. The mixture is thenslowly poured into the mold over the gel coating and cured for 2 hoursat 200°F and then 3 hours at 350°F. The completed third stage negativeproduction mold is then cooled to room temperature and demolded slowly.Additional gel coating formed of T4083A in the proportions previouslydescribed are sprayed onto the sides and bottom of the mold so that allexterior surfaces have a volume resistivity of 1 × 10.sup.⁻³ ohms-cmwith minimum film thickness of 0.005 inch. This additional coating isheated to 140°F for 2 hours and postcured for 2 hours at 350°F.

The resultant production mold has a surface coating which is uniformlyelectrically conductive and which is eminently suitable for use in highfrequency flow molding.

The resultant third stage mold 30 (FIGS. 6 and 7) has a rigid body 31with an electrically conductive surface 32 preferably extendingcompletely therearound and with a negative surface configurations 33conforming to the surface configuration of the original 12. When placedin a high frequency flow molding apparatus diagrammatically illustratedin FIG. 5 between a planar surfaced top electrode 36 and a bottom planarsurfaced electrode 37 along with a vinyl sheet 38 to be molded, thesurface configuration can easily be produced in the vinyl sheet atconventional pressures and frequency ranges. In fact, somewhat loweramounts of electrical energy are necessary. For example, a flow moldingmachine can be used for molding vinyl sheets having a thickness of 0.068inch in the form of a vamp pattern having a surface area of 100 sq.inches. The mold 30 is used and the machine operated at a pressure of 3psi, dwell time of 7 seconds, room temperature, frequency 48 megacycles,wattage 15 kilowatts drawing 1500 volts R.F. and cooling time underpressure 10 secones. Good surface texture is provided on a surface ofthe vamp. The mold 30 has a long life span with relatively inexpensivecost as compared with metal molds. The mold does not absorb heat becauseof its surface coating which is heat conductive as well as electricallyconductive causing substantially all the heat to be absorbed in thevinyl resulting in short dwell times. Moreover, the surface coating ofthe epoxy prevents absorbing of dioctyl phthalate and secondaryplasticizers from the vinyl. Shorter cooling cycles in the mold arepossible becuase of the high thermal conductivity. The rigidity and wearresistance of the molds are excellent giving high surface definition andextremely good detail in the surface coating formed.

The epoxy mold of this invention essentially has all the characteristicsof the mold without the high cost of producing a metal mold.

In an alternate form of the method of this invention, in some cases evenbetter surface definition can be obtained in flow molding by removinggases from the area between the sheet to be molded and the mold duringthe molding operation. This can be easily carried out by evacuating thearea between the platens after first forming a gas seal therebetween.Removal of air along with water vapor from the molding area enables moreuniform control of the molding operation, removes ionizable gases andwater vapor which could cause problems and prevents gas pocketing toallow free flow of material without resistance into deep cavities in themold surface.

FIG. 8 illustrates the use of vacuum and removal of gases during flowmolding. In this figure, the mold 30 previously described is positionedbetween platens 36 and 37 as previously described with a vinyl sheet 38interposed. The molding arrangement includes an encircling resilientgasket 50 attached to preferably the upper platen 36 and a solenoid flipvalve 51 connecting a vacuum line 52 to the mold area. High frequencytransmission lines 53 and 54 are attached to the platens as known in theart and a conventional hydraulic press 55 is used. Using the conditionsof the above-noted example, the vinyl sheet 38 is positioned in the moldarea with the mold attached to the lower platen 37. The platens are thenpartially closed to form a hermetic seal about the mold and vinyl sheetdue to the contact of the encircling gasket 50 with both platens. Thevacuum line 52 is then opened to create a vacuum in the molding area.Power is applied and molding carried out using conventional procedureafter which the mold is allowed to cool. The solenoid flip valve is thenopened to allow air to reenter the molding area whereby the chamber canbe easily opened.

It should be understood that the creation of a vacuum which removesgases including water vapor from the molding area is useful in highfrequency flow molding even where the electrically conductive molds ofthis invention are not used. Thus, the vacuum step has advantages evenwhere metal or other molds in place of mold 30 are used. While it ispreferred to obtain a high vacuum in the molding area during molding,any vacuum provides some advantages. Preferably a vacuum of 29 inches ofmercury is used. The vacuum can be applied only to the area between thesheet to be molded and the surface textured area of the mold if desiredrather than to the entire mold area. However, application of the vacuumto the entire mold area between the platens and surrounding the sheet isperferred.

While specific examples of this invention have been shown and described,it should be understood that many variations are possible. The plasticthermosetting mold need not be formed of an epoxy material and in somecases other thermosetting materials which have heat distortion ratesabove 250°F can be used. Such materials include other epoxys,polyesters, phenolics, silicones and combinations of these and the like.The specific fillers used to provide electrical conductivity can varywith silver, gold or platinum particles being preferred because theoxides thereof are highly electrically conductive while other metaloxides are non-conductive. The fillers can be in conventional finelydivided particle forms such as powder, flakes, spheres or irregularshpaes. In some cases, the particles can be formed with cores ofnon-conductive or conductive materials and carry desired outer coatingsof metals such as silver, gold or platinum in order to reduce cost whilemaintaining desired properties. For example, known silver coated copperparticles are suitable for use in the molds of this invention.Preferably the electrically conductive surface layer of the mold has athickness of at least 0.001 inch. The thickness can vary greatlydepending on the frequency of R.F. energy used in the molding operation.Preferably the surface layer and mold body are of the same generalfamily of plastics such as in the example given; however, differentplastic materials can be used for each so long as they are compatibleand evidence the desired characteristics of this invention.

In all cases it is preferred that the highly conductive surface layer ofthe mold have a volume resistivity no higher than 1 × 10.sup.⁻¹ ohms-cmat 20°C and more preferably be in the range of from 1 × 10³ to 1 ×10.sup.⁻⁵ ohms-cm at 20°C. Preferably the conductive layer has athickness of from 0.003 to 0.1 inch. The conductivity should besufficient to avoid RF energy absorption and heat buildup in the mold.In some cases, the entire mold can be electrically conductive althoughsurface layers as described are sufficient to achieve the resultsdesired. Preferably, the molds have hardness values of at least 45 ShoreD with minimum coating thicknesses of at least 25 microns.

In some cases, the entire mold can be electrically conductive althoughthis does not significantly increase the advantageous properties of themold. When the entire mold is electrically conductive, it is formedintegrally of a thermosetting plastic material as described above. Thusepoxys, polyesters, phenolics, silicones and the like used for eitherthe coating layer or base of the mold such as 30 is used for the entiremold body and has uniformly incorporated therein the conductive fillersof this invention as previously described. For example, a uniformlyelectrically conductive mold throughout can be formed using a siliconerubber positive mold such as 21, by pouring into the mold 21 a mixturecomprising 100 parts of Arcon epoxy T4046A and three parts by weightArcon T4046B having uniformly incorporated therein 76% by weight ofsilver flakes of a particle size ranging from 2 to 25 microns. Themolding material is mixed together and molded as previously described inthe above example and a fully uniformly electrically conductive moldidentical to mold 30 is formed except that the entire mold iselectrically conductive. In this case, the entire mold has the uniformelectrically conductive properties of the surface layer of mold 30.

While sheet molding has been described for use on shoe uppers, sheetmolding of various types can be carried out with high frequency energyfor various uses including producing of surface effects for handbags,shoes, clothing, automotive dashboards, place mats and other uses.

We claim:
 1. A method of flow molding to form a surface texture on asheet material,said method comprising, positioning a mold on anelectrode of a high frequency flow molding apparatus, said mold defininga body, a mold base and a mold outer surface carrying means fortransferring a surface configuration to a sheet to be molded, said moldbody being formed of a rigid thermosetting resin material and having anelectrically conductive layer forming a continuous path from said baseto said means, said path being formed by a plurality of finely divided,uniformly dispersed metal surfaced particle fillers, with the metalselected from the group consisting of gold, silver and platinum, andarranging a sheet to be molded, adjacent said mold means and backed by asecond electrode of said high frequency molding apparatus, applying heatand pressure to said sheet to be molded to cause transfer of saidsurface configuration to said sheet to be molded with said electricallyconductive layer acting to prevent heat buildup in said mold.
 2. Amethod in accordance with the method of claim 1 wherein said mold has asurface layer only formed of a resin material having metal surfacedparticles uniformly dispersed therein to give uniform electricallyconductive properties to said layer.
 3. A method in accordance with themethod of claim 2 in which said resin of said mold body and layer is anepoxy resin and said particles are silver particles with said layerhaving a volume resistivity of from 1 × 10.sup.⁻³ to 1 × 10.sup.⁻⁵ohms-cm at 20°C.
 4. A method in accordance with the method of claim 1wherein said pressure is in the range of from 0.5 to 100 psi and saidtemperature is in the range of from 80°F to 400° F with said heat andpressure being applied in the mold with a dwell time of from 1 to 20seconds using a frequency of from 20 to 54 megacycles at a wattage offrom 7 to 70 kilowatts and a cooling time under pressure after dwelltime of from 1 to 60 seconds.
 5. A method in accordance with the methodof claim 1 wherein a vacuum condition is created between said sheet andmold means to improve transfer of said surface configuration during saidapplication of said heat and pressure.
 6. A method in accordance withthe method of claim 1 wherein said layer has a volume resistivity offrom 1 × 10.sup.⁻³ to 1 × 10.sup.⁻⁵ ohms-cm at 20°C.
 7. In a method offlow molding wherein a mold and sheet to be flow molded are positionedbetween a pair of platens and the platens are brought together to moldsaid sheet with a surface configuration, the improvement comprising,saidmold comprising a mold body, a mold base and a mold outer surfacecarrying means for transferring a surface configuration to said sheet tobe molded, said mold body being formed of a rigid thermosetting plasticmaterial and having an electrically conductive layer forming acontinuous conductive path from said base to said means, said path beingformed by a plurality of finely divided, uniformly dispersed, metalsurfaced particles with the metal selected from the group consisting ofgold, silver and platinum and said conductive layer having a volumeresistivity of from 1 × 10.sup.⁻³ to 1 × 10.sup.⁻⁵ ohms-cm at 20°C.
 8. Amethod in accordance with the method of claim 7 wherein saidelectrically conductive layer has a thickness of from 0.001 inch to 0.1inch and said mold is formed of an epoxy having a hardness value of atleast 45 Shore D.